Field of the Invention
The invention relates to a sensor configuration for measuring the local distribution of a measured variable, in particular as a sensor seat mat for detecting seat occupancy in a motor vehicle.
In order to detect seat occupancy in motor vehicles, use is made of so-called sensor seat mats having a multiplicity of pressure-sensitive sensors which are disposed in a distributed fashion in rows and columns on the seating surface and can thereby detect the pressure distribution on the seating surface. This permits a conclusion to be drawn as to the weight of the person located on the vehicle seat. The individual pressure-sensitive sensors have pressure-dependent electric resistors. This permits an electrical measuring element to measure the pressure acting on the seating surface of the motor vehicle seat at various points. As an example, for this purpose a constant current is fed into an individual row of the resistor configuration in matrix form, and the voltage drop across these rows and one of the columns is measured. Voltage drop across the associated resistor permits a calculation of the pressure acting at this point on the seating surface.
It is problematical in this case that the constant current fed in does not flow off only via a single pressure-dependent resistor, but also via parallel current paths which are formed by other pressure-dependent resistors of the sensor configuration. These parasitic currents falsify the measurement result. It is therefore necessary when measuring to ensure that the same voltage levels are present in each case at the other rows and columns, in order to avoid parasitic currents.
The calculation of the pressure acting on the seating surface from the measured resistance is performed with the aid of a prescribed characteristic line. It is preferable to use a low-resistance operating range of the characteristic line, since otherwise substantial errors could occur due to the influence of leakage currents. On the other hand, it must necessarily be avoided that a prescribed minimum resistance is undershot, since otherwise the above-described suppression of parasitic currents is rendered difficult. The disadvantage of this conflict between design targets is that the available operating range of the prescribed characteristic line cannot be utilized efficiently, since the pressure-dependent resistance is not permitted to drop below a prescribed limiting value. Therefore, only the high-resistance operating range with a correspondingly large measurement error can be used. Thus, the measurement error in the case of measurement in the range from 500 kxcexa9 is up to 80%, for example.
It is accordingly an object of the invention to improve the above-described known sensor seat mat to the effect that it is possible to suppress parasitic secondary currents in conjunction with a measurement error which is as low as possible.
With the foregoing and other objects in view there is provided, in accordance with the invention, a sensor configuration for measuring the local distribution of a measured variable, comprising a plurality of sensor elements disposed in a distributed fashion and interconnected in an array. An electric response thereof is a function of the local value of the measured variable. At least one of the sensor elements has a series element independent of the value of the measured variable and a measuring element dependent on the value of the measured variable.
In one embodiment, the measuring element is a pressure-dependent resistor with a nominal resistance of between 100 kxcexa9 and 500 kxcexa9 and the series element is an ohmic resistor with an ohmic resistance of between 1 kxcexa9 and 10 kxcexa9. However, it is also possible to use as a measuring element a temperature-dependent resistor or a capacitor whose capacitance is a function of that measured variable.
In another embodiment, the sensor elements are disposed in a substantially planar fashion in rows and columns including two films disposed substantially parallel to one another made of electrically insulating material, on whose mutually facing lateral surfaces a plurality of conductor tracks are applied. A high-resistance layer which is disposed between the two films in each case has cutouts in the region of the points of intersection of the conductor tracks. The cutouts can be filled with a resisting material. Alternatively, resisting material can be disposed between the conductor tracks at the points of intersection of the conductor tracks without the need for cutouts.
In another embodiment, the above-noted sensor configuration includes a plurality of electrical terminals connected to the sensor elements for transmitting the electric response to a measuring instrument and/or to seat occupancy device in a motor vehicle.
The above-described sensor configuration can be used in combination with a seat occupancy device in a motor vehicle.
The invention is based on the general technical teaching of connecting the pressure-dependent resistor in series with a pressure-independent series resistor, in order to prevent the total resistance from falling below the prescribed minimum value. The operating range of the characteristic line can be more effectively utilized in this way, and therefore the sensor configuration can be operated in the more accurate low-resistance range. The measurement error can thus be minimized on the basis of the preferably logarithmic characteristic line.
However, the invention is not limited to the pressure acting on the seating surface as regards the measured variable to be detected. Rather, the principle according to the invention can also be used to detect other measured variables such as, for example, the temperature.
Neither is the invention limited to pressure-dependent resistors with respect to the sensor elements to be used, but it can also be implemented with other components whose electric response depends on the measured variable to be detected. Mention may be made, for example, of capacitive components in which the capacitance is a function of the dielectric properties of the sensor environment, i.e. whose capacitance is dependent on the measured variable.
In the preferred embodiment of the invention, the individual sensor elements contain a series circuit composed of a series element independent of the measured variable, and a measuring element dependent on the value of the measured variable. The measuring element is preferably a pressure-dependent resistor with a nominal resistance of preferably between 100 kxcexa9 and 500 kxcexa9. The series element is, by contrast, preferably an ohmic resistor whose nominal value is independent of the value of the measured variable to be detected and is preferably in the range of between 1 kxcexa9 and 10 kxcexa9.
The sensor elements are preferably disposed in a substantially planar fashion in rows and columns, the rows preferably being aligned at right angles to the columns. It is, however, also possible to arrange the individual sensor elements inside a space lattice, in order to determine the spatial distribution of the measured variable to be detected.
In the preferred embodiment, the sensor configuration according to the invention has two films, disposed substantially parallel to one another, of an electrically insulating material. A plurality of conductor tracks is applied to the mutually facing lateral surfaces of the films. The application of the conductor tracks can be performed in this case by printing or spraying, for example, but other methods of production are also possible. In this case, a high-resistance layer is disposed between the two films and in each case has cutouts in the region of the points of intersection of the conductor tracks. In this way, the films disposed adjacent one another can be pressed together in this region, and this leads to contacting of the conductor tracks. The cutout of the electrically insulating material is preferably also filled with a low-resistance resisting material, in order to increase the electrical resistance between the adjacent conductor tracks in the case of contact as well, and thereby to permit utilization of the entire operating range of the characteristic line.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a sensor configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.